CASE STUDIES ORGANIC RANKINE CYCLE
Organic is good for you
The Organic Rankine Cycle’s (ORC) principle is based on a turbogenerator working as a normal steam turbine to transform thermal energy into mechanical energy and finally into electrical energy through an electrical generator. Instead of steam from water, the ORC system vaporises an organic fluid, characterised by a molecular mass higher than water, which leads to a slower rotation of the turbine.
ORC technology is being used in a number of energy applications, mostly in biomass and geothermal schemes, but great rises in solar and heat recovery applications are also expected. In a typical biomass combined heat and power
CHP plant in biomass applications
plant the process is based on the following thermodynamic cycle:
• •
•
A heat source heats thermal oil to a high temperature – typically about 300°C – in a closed circuit
The hot thermal oil is drawn to and from the ORC module in closed circuit. In the ORC it evaporates the organic working fluid of the ORC in a suitable heat exchanger system (pre-heater and evaporator)
Organic vapour expands in the turbine, producing mechanical energy, which is further transformed into electrical energy through a generator
The vapour is then cooled by a fluid in a closed circuit and condensed. The water warms up at about 80-90°C and it is used for different applications requiring heat
•
The condensed organic fluid is pumped back into the regenerator to close the circuit and restart the cycle.
The ORC cycle has a high overall energy efficiency: 98% of incoming thermal power in the thermal oil is transformed into electrical energy (around 20%) and heat (78%), with extremely limited thermal leaks – only 2% due to thermal isolation, radiance and losses in the generator.
District heating
Drying
Low temperature thermal oil loop
Refrigeration
Biomass powered boiler (pruning of branches, marcs, husk, wood chips, saw dust, bark)
High temperature thermal oil loop
Heat sink
ORC systems will prove to be more reliable than steam turbine technology or gasification systems
be working at pressures of only several bar. The by-product from electricity generation
using ORC technology is hot water at 90°C; this is far more useful in a building services setting than steam, since it can easily be used for heating. The electrical efficiency can also be raised to around 21% to 22%, but this will reduce the water temperature by-product to 40°C to 50°C, which is less usable – unless your application has a high demand for hot water in kitchens, showers, and so on. The technology is far from widespread yet,
in the UK, but BAA has opted for a similar biomass-fuelled CCHP plant at its new energy centre for Heathrow Terminal 2, which is currently being completely redeveloped prior to re-opening in spring 2014. The energy centre will also contribute base heating and power to Terminal 5 in advance of the £2.5bn Terminal 2 becoming operational. The 10 MW system will deliver a thermal
38 CIBSE Journal December 2012
capacity of 8 MW (75% heat and 25% to chillers, providing heating and cooling to Terminals T2a and T2b and heat only to Terminal T5) and drive a Turboden ORC electricity generator, rated at 1.8 MWe. The plant will enable Heathrow to reduce CO2 emissions at the airport by at least 13,000 tonnes each year, compared with producing the same output from natural gas. The aim is to reduce Heathrow’s carbon footprint by 34% by 2020, with the new Terminal 2 facility using 40.5% less than requirements to meet Building Regulation Part L 2006 compliance. ‘ORC systems will prove to be more
reliable than steam turbine technology or gasification systems,’ says Heffernan. ‘They are only just starting out in the UK and we haven’t got back into investment mode on large projects, but they will appear on schemes in the future.’ William Rankine was way ahead of his time. CJ
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